The invention relates to a multi-screw extruder for the continuous processing and/or working of a bulk material, in particular a powder, grain or flaked product, and to a method for filling the processing spaces of such a multi-screw extruder.
Multi-screw extruders for processing and/or working bulk material are known in the art. A special group of multi-screw extruders has parallel abutting screws divided into a first process space and second process space for processing the product inside the extruders. In areas where the processing elements are tightly meshing conveying elements, there is essentially no connection to enable an exchange of material between the first process space and the second process space at least in this area. At most gases and traces of the most finely distributed product can get from one to the other process space. The impossibility of any product exchange between the two process spaces is particularly disadvantages in the feed zone of such a multi-screw extruder with several process spaces if only one of the process spaces can be filled with product via the feed hole of the extruder. The other process space(s) in this feed zone is/are filled with extremely small quantities of the product, if any. This translates into a waste of process space volume and a limitation of product feed capacity of the extruder. This is particularly disadvantageous in the case of a ring extruder with several screws arranged as a rim in a hollow space of its casing, which run parallel to the axial direction of the casing and form an inner process space inside the rim along with an outer process space outside the rim. Only the outer process space outside the rim is here filled with product via the feed hole, while the inner process space remains unused in the area of the feed hole. In the end, this feed limitation allows only a fraction of the potential throughput to be realized with this ring extruder.
One potential solution to this is offered by DE-196 04 228, which discloses such a ring extruder for the continuous processing of free-flowing materials. In this case, the aforementioned problem of feed limitation is resolved by forming at least one opening in the screw rim by omitting the conveying properties of at least one processing element in the area of the material feed hole in the process space of the ring extruder. Instead of a conveying screw element, a spacer sleeve with a smooth outer cylinder wall is used in this area on the mandrel of at least one of the screws of the rim in the area of the material feed hole. While this enables a material exchange between the outer and inner process space in the area of the feed hole, it does so at the cost of the conveying properties in this area. As a result, a portion of the product passed through the material feed hole remains in the dead spaces of the feed zone, so that optimum use is still not made of the volume of the inner process space.